Mass spectrometry (MS) is an analytical methodology used for quantitative elemental analysis of samples. Molecules (often referred to as analytes) in a sample are ionized and separated by a spectrometer based on their respective masses. The separated analyte ions are then detected and a mass spectrum of the sample is produced. The mass spectrum provides information about the masses and in some cases the quantities of the various analyte particles that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte.
One type of mass spectrometry is time-of-flight (TOF) mass spectrometry. TOF mass spectrometry is used to form a mass spectrum for ions contained in a sample of interest. In a TOF mass spectrometer, ion packets are provided to a flight region of the instrument. The ions separate according to their mass-to-charge ratios, m/z, and their arrival times Ti are recorded. According to a known TOF method, a sample is divided into packets of ions that are launched along the propagation path using a pulse-and-wait approach. In releasing packets, one concern is that the lighter and faster ions of a trailing packet will pass the heavier and slower ions of a preceding packet. Using this so-called pulse-and-wait approach, the release of an ion packet is timed to ensure that the ions of a preceding packet reach the detector before any overlap can occur. Thus, the periods between packets are relatively long. If ions are being generated continuously, only a small percentage of the ions are actually detected. A significant amount of sample material is thereby wasted. The loss in efficiency and sensitivity can be reduced by storing ions that are generated between the launching of individual packets, but the storage approach carries some disadvantages.
The inefficiencies of the known pulse-and-wait methods have led to various techniques of sending ions in pulses from a stream of ions for a sample to the detector. In one known method, the ions are pulsed using a pseudo-random pulsing sequence. Because the pulsing sequence is known, the mass spectrum can be recovered by an inverse transform. One example of pseudo-random pulsing of ions in TOF mass spectrometry applications is dithered multi-pulsing, in which a small interval is added randomly to a nominal pulse interval, Measured peaks can be assigned to their respective pulses of origin by statistical analysis, Another example is Hadamard transform TOF, in which a continuous ion beam is modulated between two detectors according to a pulse sequence typically running at 10 kHz-1 MHz. This results in a so-called 100% duty-cycle measurement. It is possible, however, to modulate the continuous ion beam “on” and “off” at a single detector with 50% duty cycle. The resulting spectrum looks like noise, but the original mass spectrum can be recovered using the inverse Hadamard transform.
While these other known methods and attendant components do provide an improvement in efficiency over the pulse-and-wait methods, they generally require additional hardware and significant data processing resources. This results in a more complex TOF MS instrument. Moreover, by these known pulsing methods, artifacts may be introduced in the process of reconstructing lost information. As such, a unique solution is not guaranteed, and several different recovered mass spectra may be consistent with the measured data, which can result in reduced instrument accuracy.
What is needed, therefore, is a method and apparatus for analyzing ions that overcomes at least the drawbacks of the known methods described above.